In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Currently the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit (Tx) diversity (TxD) in the context of High-Speed Uplink Packet Access (HSUPA). With uplink transmit diversity user equipments that are equipped with two or more transmit antennas are capable of utilizing all of them. FIG. 1 is a schematic overview depicting an example of a user equipment using uplink TxD. The uplink TxD is achieved by multiplying a signal s(t) with a set of complex weights vi. The weights may e.g. define level of amplification, precoding or similar. Note that i=1 . . . N where N denotes the number of transmit antennas, a1-aN. The rationale behind uplink transmit diversity is to adapt the antenna weights so that the user equipment and network performance are maximized in terms of bitrate. Depending on implementation in the user equipment, the antenna weights may be associated with different constraints. Within 3GPP two classes of constraints are considered. Firstly, a class called switched antenna diversity, where the user equipment at any given time-instance transmits from one of the transmit antennas only. Thus if vi≠0, vj=0 for all j≠i. Secondly, a class called beamforming, where the user equipment at a given time-instance can transmit from more than one transmit antenna simultaneously. While switched antenna diversity is possible for UE implementations with a single Power Amplifier (PA) the beamforming implementation may require one PA for each transmit antenna. Switched antenna diversity may be seen as a special case of beamforming where one of the two antenna weights is 1, i.e. switched on, and the other one is 0, i.e. switched off.
Because the radio propagation channels, h1,1-hN,M, from the multiple transmit antennas a1-aN to the receiver antennas, b1-bM, differ, the user equipment and network performance depends on how the multiple antennas are used. A radio network node, e.g. a radio base station, comprises a combiner that combines all the received signals. By using switched antenna diversity or beamforming a gain may be achieved compared to transmission from a single transmit antenna, for example by transmitting from the transmit antenna with the best radio propagation conditions as often as possible. This is the main idea behind these uplink transmit diversity schemes.
In current radio communications networks, e.g. in WCDMA, user equipments in idle state monitor the system information of a radio base station within range to inform itself about candidate radio base stations in the service area etc. When a user equipment needs access to services, the user equipment sends a request over the Random Access Channel (RACH) to a Radio Network Controller (RNC) via the most suitable radio base station, typically the one with the most favorable radio conditions. Since the uplink propagation is only approximately known, the user equipment gradually increases the transmission power of a random access preamble until either the random access preamble has been acknowledged via the downlink Acquisition Indicator Channel (AICH), or the maximum number of attempts of transmitting the random access preamble has been reached. Upon acknowledgement of the random access preamble, the RACH message is sent. After admission control at the RNC, the RNC initiates the connection via the most suitable radio base station if there are available resources.
For each transmission the user equipment selects a random access preamble at random among up to 16 available random access preambles. Each preamble of length 4096 chips is constructed from signature sequences that are scrambled with a scrambling code. The signature sequences are e.g. 256 repetitions of orthogonal 16-chip Hadamard sequences. The random access preamble is typically detected using a matched filter that is matched to the random access preamble signal. The power of the output of the matched filter is typically compared to a detection threshold for random access preamble, and the random access preamble is acknowledged when the output exceeds the detection threshold.
Uplink transmit diversity schemes, such as switched antenna diversity or beamforming, require some knowledge or information about the radio transmission conditions for the multiple transmit antennas. That information may be obtained by explicit feedback from the radio base station, or by measuring how successful previous transmissions have been.
The problem is that the random access preamble, also referred to as the RACH preamble, is the first to be transmitted by a user equipment in WCDMA. That means that the radio communications network and the radio base station is unaware of the user equipment, so no feedback is received, and the user equipment has no immediate previous transmissions to use to select transmit antenna or antenna weights. Also, before a successful random access the radio communications network does not know if the user equipment has uplink transmit diversity capability or not. Therefore it is hard to benefit from the uplink transmit diversity that multiple transmit antennas may give.